The present invention relates to an optical pickup for an optical disk having multiple recording layers, and more particularly to, an optical pickup for an optical disk having multiple recording layers which are simultaneously scanned for recording or reading out information on or from the multiple recording layers of optical disk.
The optical disk is widely known as an optical recording medium, which has multi-optical layers in which two or more recording layers on which information is recorded are overlapped. Also, an optical pickup from which the recorded information is read out by simultaneously scanning the multiple recording layers thereof is well known, as compared with the optical disk having such multiple recording layers.
FIG. 1 shows a conventional optical pickup having dual recording layers. An optical beam generated from an optical source 1 is collimated by a collimating lens 2. The collimated beam is reflected by a beam splitter 3 and transmitted to a Fresnel lens 4 as a diffraction element. Then, the transmitted light is incident to an objective lens 5. Thus, a plurality of beams diffracted by Fresnel lens 4 are incident to objective lens 5, so that beam spots are respectively formed on recording layers 7 and 8 of optical disk 6.
The beam reflected from recording layers 7 and 8 of optical disk 6 passes through beam splitter 3 via objective lens 5 and Fresnel lens 4. The reflected beam having passed through beam splitter 3 is detected as an electrical signal by an optical detector 11 via a condensing lens 9 and a sensor lens 10.
In the conventional optical pickup as described above, Fresnel lens 4 and objective lens 5 are jointly set in a body tube 12. Focus driving coils 13 are attached to both sides of body tube 12 and permanent magnets 14 for generating magnetic flux are arranged on the exterior of body tube 12 near focus driving coil 13. Focus driving coils 13 vertically moves together with body tube 12 by the focus error signal extracted from the signal detected by optical detector 11. That is, objective lens 5 is driven together with Fresnel lens 4 so as to compensate a focusing error of objective lens 5 with respect to optical disk 6, which is caused due to the vertical vibration of the optical disk.
In this structure where Fresnel lens 4 is driven together with objective lens 5, since two beam spots respectively formed on recording layers 7 and 8 of optical disk 6 move in unison, there is a constant interval between the beam spots. The interval between recording layers 7 and 8, however, is varied due to variations in the thickness of the recording layers, caused by molding imperfections or deformation such as warpage.
Thus, in the above-described conventional optical pickup, the focal positions of beam spots formed on the multiple recording layers cannot be exactly adjusted, so that the optical pickup cannot be used for an optical disk having the multiple recording layers.
Also, according to the conventional optical pickup, since the two beam spots formed on the multiple recording layers of the optical disk originate from a single optical source, information cannot be independently recorded on two recording layers by an optical modulation method, so that the optical pickup can be applied only for reproduction and not for recording.